The present invention relates to a device for braking an electric drive motor of a hand-held working tool, especially of a chain saw, comprising a safety brake for immediately stopping the working tool and an independent coasting brake for coasting the working tool to a stop after switching off the drive motor.
It is known from devices of the prior art for hand-held working tools, especially for portable electric chain saws, to provide two independent brake systems consisting of a mechanical as well as an electrically operated brake.
For example, from German Offenlegungsschrift 35 30 685 a brake system with two independently operating brakes is known. A mechanical brake system serves as a safety brake and after actuation brings the chain of the chain saw to a standstill within a very short time. The second brake system is an electric coasting brake which speedily stops the chain of the chain saw upon release of the on-switch, but within a considerably longer amount of time as compared to the safety brake. The electric braking is carried out by reversing the polarity of a field coil and by short-circuiting with the rotor coil.
The known brake systems have the disadvantage that the electric coasting brake is of a complicated construction and is thus expensive to manufacture.
When using an electric coasting brake, it is desired, on the one hand, to bring the chain to a standstill within a shortest possible amount of time which requires high braking current and thus a sufficiently large dimensioning of the drive motor acting as the brake. On the other hand, for limiting these currents while simultaneously providing effective braking, a complicated electric control circuit must be realized which is also expensive.
Furthermore, upon reversing polarity of the electric motor for switching from drive mode to braking mode an electric arc results within the switch for reversing polarity which can irritate or distract the operator and can result in dangerous situations.
It is therefore an object of the present invention to provide a coasting brake that is comprised of few components and provides a high functional reliability with minimum wear.